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Rollback is the term for backtracking a server from one state to a previous state, to undo a mistake/problem. Essentially, before making any large changes, you make a restore point that will ensure you have a working server if you make a misstep. This feature is only available in Virtual Machines. In the case of a real physical server, rollback cannot be done, but all mistakes can be undone with diligence and the rescue procedure.

For example, before installing Hiawatha/PHP/MySQL, which has numerous dependencies and daemons, you may consider making a snapshot. If the install process gets botched, return to the exact moment before you made these changes and try again. Likewise, after successfully installing this software, make another snapshot in case changes you make afterward mess up the server. With this, you never have to re-do the webserver steps (and you can likewise remove outdated restore points).

Another good use of this is right after building a kernel, but BEFORE booting it. menu makeconfig a new kernel, make all && make modules_install and after it is complete and files are copied--take a snapshot. If Linux fails to boot, you can restore, make menuconfig, add the missing components, rebuild--all in minimal time.

Virtualbox snapshot / restore

Using Virtualbox, this process is exceedingly easy.

Create a snapshot before a large change
Describe what changes were made successfully
Restore when desired